There could be a split in painkillers:
Natural, which are things like Aloe, used to alleviate pain slightly, and do not form addictions. Yes, i know, it's for (sun)burns, but it's a start.
Alchemic/Reactory, where a basic painkiller/salve is made from a bunch of plants mashed up together. Nonaddictive, but not always safe- some salves create problems as well, such as causing nausea, drowsiness, thirst, or what-have-you. They're kind of nastier the more powerful they are- anti-necrotic salve, for instance, could cause pus, nausea, and the like.
Finally, there is the much debated Drugs, which WOULD be addictive, and could cause problems, and might not even be useful (for instance, a highly addictive substance that causes necrosis and drowsiness) or be only lightly addictive and possibly not even affect those with strong will.
For dwarves, most if not all of these substances are criminal and owning them is a crime. However, some, such as those that give slight happiness, are lightly addictive, and don't cause large problems would simply be frowned upon (such as tobacco chew and cigarettes are in today's culture).
This "addictive" system could be added to many things: for instance, different beers with addictive plants would be craved by dwarves, and while only a super-strong addictiveness would drive them mad without it, it produces a bad thought if they go so long without the substance, then another one when they do recover from addiction.
Addicting tag:
[ADDICTIVENESS:x:y:z:z2:z3]
x is the overall addictiveness of the substance, where 1-20 is very low, 21-40 is low, 41-60 will be addicting to almost everyone, 61-80 is highly addicting, and 81+ is extremely addicting and requires a long recovery period (up to 4 bad thoughts in the cycle of recovery.)
y is the overall initial happy thought given and the bad thoughts involved with recovery. The same scale as above works:
Weak, which gives an initial minor happy thought and has a minor bad thought when in recovery.
Low Strength, which is a minor happy thought and a bad thought.
Average, which is a happy thought and big unhappy thought.
High is a big happy thought and a very unhappy thought.
Extreme is a very happy and very very unhappy thought.
z is the decrease in happines from repeated use, again, with the scale above:
0=No Drop
Slight Drop: The drug slowly loses it's effect, giving the same level of inital happy thought for a long time.
Low Drop: The drug loses it's effect gradually, losing it's power in a good number of uses.
Average: The drug loses it's effect over time, losing it's power in a few uses.
High: The drug quickly loses it's effect, it's power diminished after 2-3 uses.
Extreme: The drug loses it's effect very quickly, and never feels as powerful as the first time.
z2 is the minimal effectiveness of the drug: repeated uses of the drug will not drop below this level of happy thought (same scale as y)
z3 is the recovery time, and determines time between "withdrawl attacks".
0 is an absence of withdrawl attacks: they simply do not happen, though this means that recovery is impossible
1 is a very short time, roughly a week.
2 is roughly a month
3 is a season (3 months)
4 is a year
each number is after 4 increases the time by 1+ year.
This is as unbiased a drug system as I could think of, and while this would allow for... many morally darker mods, I don't think that the system itself is as such. Yes, we'd see a bunch of "marijuana and cocaine mods" but we'd also see additions in larger mods of drugs from the spectrum- and it's muchly up to the player how their dwarves work anyway. Their base morals think of even most low-level addictive substances as very criminal and the very addictive ones as unthinkable (which prevents the dwarves from brewing, eating, or making anything with said substances).